Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
FIG. 1 illustrates a powertrain 10 with a transmission 12. The flow of mechanical power is illustrated by solid lines, while dashed lines indicate the flow of information signals, which may be conveyed electrically or hydraulically. Power is generated by engine 14 and conveyed to a transmission input shaft 16. A torque converter 18 and gearbox 20 modify the speed and torque at which the power is delivered to match vehicle requirements while permitting engine 14 to run at a suitable crankshaft speed. Power flows from the torque converter to the gearbox via a turbine shaft 22. Driveshaft 24 transfers power from transmission 12 to differential 26. Differential 26 distributes the power between drive wheels 28 and 30 while allowing slight speed differences such as when turning a corner. Some transmissions, such as front wheel drive transaxles, may include the differential in the same housing with the gearbox and torque converter. In such transmissions, power transfer to the differential may utilize gears or chains as opposed to a driveshaft. In some vehicles, a transfer case may be interposed between the transmission and differential to transfer some power to additional wheels.
Transmission controller 32 adjusts the state of transmission 12 based on various inputs, including vehicle speed measurements, driver torque demand as indicated by accelerator pedal position, and a shift selector. Controller 32 may adjust the state of the transmission by sending electrical signals to valve body 34. In response to these signals, valve body 34 adjusts the pressure in hydraulic circuits to engage particular clutches, such as clutches within gearbox 20 and a bypass clutch within torque converter 18.
FIG. 2 schematically illustrates a torque converter 18. Impeller 40 is fixed to input shaft 16 and supported by transmission case 42. In operation, the space enclosed by these components is filled with transmission fluid. Turbine shaft 22 is drivably connected to turbine 44. A torsional damper may be interposed between turbine 44 and turbine shaft 22 to isolate gearbox 20 and other driveline components from engine vibrations. Stator 46 is coupled to transmission case 42 via one way clutch 48. When the turbine shaft is stationary or rotating slowly relative to transmission input shaft 16, one-way-clutch 48 holds stator 46 stationary. Rotation of impeller 40 forces fluid to move between the impeller, the turbine, and the stator. The fluid exerts a hydro-dynamic torque on the turbine 44. Stator 46 provides a reaction force such that the torque on turbine 44 can be greater than the torque on impeller 40. When the speed of turbine 44 approaches that of impeller 40, fluid tends to flow around the centerline 50, causing one-way-clutch 48 to overrun. The chamber 52 that includes the turbine, the impeller, and the stator is called the hydro-dynamic chamber.
To improve power transfer efficiency once the vehicle reaches a sufficient speed, the controller may engage bypass clutch 54 to selectively couple transmission input shaft 16 to turbine shaft 22. Clutch pack 56 includes one or more plates that rotate with input shaft 16 interleaved with one or more plates that rotate with turbine shaft 22. To engage the clutch, pressurized fluid is routed to apply chamber 58 forcing piston 60 to compress clutch pack 56. When the pressure is released, spring 62 forces piston 60 away from the clutch pack. Fluid pressure in balance chamber 64 also tends to push piston 60 away from clutch pack 56. Balance chamber 64 may be filled with fluid at low pressure such that fluid pressurization due to centrifugal forces are canceled out. The controller may partially apply clutch 54 such that the speed difference between the input shaft 16 and turbine shaft 22, called the slip, is a desired amount. During partial clutch application, some torque is transferred by clutch 54 and the remainder of the input torque is transferred hydro-dynamically via the impeller, stator, and turbine. Precise control of the torque capacity of clutch 54 is required to maintain a desired slip.